A study dealing with the ground and excited state behavior of diazabicyclo(3.1.0)hexenes, N-aroylaziridines and 2H-azirines was initiated. The photo- and thermal reactions encountered with the fused aziridines were readily accounted for by carbon-carbon fission of the aziridine ring to form 1,3-dipolar intermediates (azomethine ylides). The azomethine ylides were found to undergo 1,3-dipolar cycloaddition reactions with homo and hetero multiple bonds to give a variety of heterocyclic rings. The thermal and photochemical expansion reactions of unsaturated 2H-azirines have also been examined. The azirines undergo thermal rearrangement by rupture of the C-N single bond to give a vinyl nitrene which undergoes subsequent cyclization with the adjacent double bond. The azirine derivatives were also found to undergo photochemical reorganization via transient nitrile ylide intermediates which can be trapped with external dipolarophiles. The thermal interconversion of 4-acylisoxazoles was shown to proceed by cleavage of the O-N bond to produce a reactive acyclic intermediate which undergoes bond rotation and subsequent reclosure to an isomeric isoxazole.